Turbine blades are the primary elements of wind turbines for converting wind energy into electrical energy. The working principle of the blades resembles that of an airplane wing. The blades have the cross-sectional profile of an airfoil such that, during operation, air flows over the blade producing a pressure difference between the sides. Consequently, a lift force, which is directed from a pressure side towards a suction side, acts on the blade. The lift force generates torque on the main rotor shaft, which is geared to a generator for producing electricity.
The lift force is generated when the flow from the leading edge to the trailing edge creates a pressure difference between the top and bottom surfaces of the blade. Ideally, the flow is attached to both the top and bottom surfaces from the leading edge to the trailing edge. However, when the angle of attack of the flow exceeds a certain critical angle, the flow does not reach the trailing edge, but leaves the surface at a flow separation line. Beyond this line, the flow direction is generally reversed, i.e. it flows from the trailing edge backward to the separation line. A blade section extracts much less energy from the flow when it separates.
Flow separation depends on a number of factors, such as incoming air flow characteristics (e.g. Reynolds number, wind speed, in-flow atmospheric turbulence) and characteristics of the blade (e.g. airfoil sections, blade chord and thickness, twist distribution, pitch angle, etc). The detached-flow region also leads to a decrease in lift and an increase in drag force, mainly due to a pressure difference between the upstream attached-flow region and the downstream detached-flow region. Flow separation tends to be more prevalent nearer the blade root due to the relatively great angle of attack of the blade flow surfaces in this region as compared to the blade tip.
Hence, in order to increase the energy conversion efficiency during normal operation of the wind turbine, it is desired to increase the lift force of the blades while decreasing the drag force. To this purpose, it is advantageous to increase the attached-flow region and to reduce the detached-flow region by moving flow separation nearer the trailing edge of the blade. This is particularly desirable nearer to the blade root in order to increase the overall lift generated by the blade.
It is known in the art to change the aerodynamic characteristics of wind turbine blades by adding dimples, protrusions, and/or other structures on the surface of the blade. These structures are often referred to as “vortex generators” and serve to create one or more vortexes that enhance the momentum of the flow to overcome an adverse pressure gradient and prevent separation. As such, the vortex generators are configured to prolong the attached flow region and thus optimize aerodynamic airflow around the blade contour.
Conventional vortex generators are typically constructed of plastic and contain one or more “ribs” or shaped structures connected to a base that is attached to one of the flow surfaces of the turbine blade. In the past, such vortex generators had to be custom designed to fit in an exact area on the blade for which it is intended to be used, as the ribs are typically stiff and cannot easily conform to blade curvature. Thus, the manufacturing cost associated with customizing vortex generators is expensive due to higher part count and tooling costs since unique molds must be built for each part.
Accordingly, the industry would benefit from improved vortex generators that address the aforementioned issues. More specifically, universal vortex generators that could be used with nearly any blade surface would be advantageous.